When I used an iMac G3 and the iBook G3 (with double the CPU clock, more RAM, and 8x the video memory, still ran terrible), I used to take .FLV files and convert them to .mov using FLV Crunch.
Decent app, I don't know what it does to bitrate, but on any Gx you won't enjoy any good bitrate, or 720p+ at all. It might toss it inside a MPEG2 codec inside a .mov container...in fact, let's find out right now, since I never knew before...Ahhh turns out it ups the bitrate a little and uses H.263.
Obviously any videohead would have heard of SUPER©, and it's pretty good, but it's just a front end for ffmpeg (and like two others) encoders. Wouldn't use them for professional production, but they work fine for our use. For those who don't know, most converters are just frame copiers and redump it into a new container or a new codec (Well...yeah...sort of). HandBrake is neat but it doesn't work well on anything PPC and it only seems to crunch H.264.
One question I'd like to know is if I take some FLV container file which has a bitrate of 1Mbps, if I should use that same 1Mbps bitrate if I convert to MPEG Layer 10/AVC/.mp4 with H.264. H.264 apparently can do the same quality with less bitrate, but I don't know how to quantify that (ie input 1Mbps FLV output 768Kbps). H.264 is actually quite good, but it's not the absolute best. I use it because it works fine on any decent Pentium platform, iOS or Android devices, and anything with a Core processor (Mac) or up.
Obviously if you want to _play_ video well on a PPC, you'd be best to pre-process it on a more powerful machine, which sort of ruins the point....